AFAIK, AC motors are totally frequency dependent. That would be the 60 cycle standard in North America, or the 50 cycle standard in Europe.
Hence, the evolution of the variable frequency drive for AC motors.
The operating principle of AC and DC motors differ. The DC motor has a commutated rotor, whereas the typical AC motor does not. The DC motor will switch its own windings sequentially depending on the position of the brushes and commutater, but it operates on a zero frequency current.
The AC motor does not switch this way, but relies on the "external clock" imposed by the power grid frequency: thus it's magnetic poles switch polarity at a given rate as the current rises and falls, and this determines the pulse rate of the motor. There is a fair degree of "slippage" in any AC motor because they should rotate at 1800rpm theoretically, but typically only achieve 1725 to 1750.
Good quality variable frequency drives do rely on encoder feedback to maintain a given rpm. This is actually more accurate than running the motor "off the powerline", where you never know what speed you are getting, depending on the load.